1. Field of the Invention
This invention relates generally to the design of electro-optic imaging systems, and more particularly, to the “end-to-end” design of systems used to image objects in which different color channels of the object are correlated.
2. Description of the Related Art
Electro-optic imaging systems typically include an optical subsystem (e.g., a lens assembly), an electronic detector subsystem (e.g., CCD detector array) and a digital image processing subsystem (e.g., typically implemented in dedicated chips or software). Traditional methods for designing these systems generally involve two discrete stages. First, the optical subsystem is designed with the goal of forming a high quality intermediate optical image of the source (subject to cost, physical and other non-imaging constraints). Next, after the optical subsystem has been designed, the digital image processing subsystem is designed to compensate for remaining defects in the sampled intermediate optical image.
In many imaging applications, the objects of interest have many spectral components. Traditionally, the optical designer optimizes the lens design parameters to minimize a variety of optical aberrations so as to produce a high quality optical image at a single image plane. Applications involving imaging of spectrally broad sources require that these aberrations be minimized over a range of wavelengths dependent on the spectral sensitivity of the detector subsystem. In such applications, the dispersion found in optical glasses and plastics makes it difficult to focus all wavelengths at the same point. Without correction, the location of the “in-focus” image plane will vary for different color bands or “channels.” The image for the red channel might be in focus at one location, the image for the green channel at another location and the blue channel image at yet a third location. Conversely, positioning the detector array at one fixed location means that one color channel may be in focus while the others are out of focus. This variation of best focus with wavelength is known as axial chromatic aberration.
The standard practice to minimize axial chromatic aberrations involves choosing lens materials with suitable dispersions to balance the aberrations. For example, the first and third lens elements (positively powered elements) in a triplet lens system often have very high Abbe numbers (Crown glasses) to minimize positive axial chromatic aberration. The second negative lens element is constructed with a low Abbe number glass material (Flint glasses) so as to impart strong negative chromatic aberration balancing the positive chromatic aberrations of the first and third lens elements. The traditional approach attempts to bring all color channels into sharp focus at the same image distance. However, the resulting lens designs can be relatively complex and expensive.
U.S. Pat. No. 5,748,371 to Cathey, Jr. et al. describes a different approach. A phase mask is introduced in the optical subsystem so that the aggregate modulation transfer function (MTF) averaged over all color channels is relatively insensitive to shifts in the image plane. Instead of being “sharply in focus” at one location and then degrading fairly rapidly to “extremely out of focus” as the image plane is shifted away from the optimal image distance, the optical subsystem in Cathey is designed so that it is “moderately out of focus” over an extended range of image plane positions. That is, the full color image is always somewhat blurry but does not get significantly better or worse as the location of the image plane changes. This effect is used to extend the depth of focus of the overall system. However, one major drawback is that the image is always somewhat blurry. In other words, the MTF suffers from low contrast.
U.S. Pat. No. 7,224,540 to Olmstead et al. and U.S. Pat. No. 5,468,950 to Hanson describe a different approach. In these examples, the objects are limited to the special class in which the images for all color channels are the same. That is, the red channel image must be the same as the green channel image, must be the same as the blue channel image. Both patents concentrate specifically on the imaging of black and white bar codes. For this special case, the black and red bar code (i.e., the image of the black and white bar code in the red color channel) is the same as the black and green bar code, is the same as the black and blue bar code. As a result of this property, any one color channel is sufficient to obtain a complete image of the object. Accordingly, the optical subsystem is designed to enhance axial chromatic aberration. Different color channels focus at different image distances. The color channel that is in best focus for the actual object distance is used as the image of the object. Enhancing the axial chromatic aberration extends the effective depth of field of the overall system since it extends the range over which at least one of the color channels will be in focus. However, this approach is limited to this special class of objects and assumes that the entire object is located at a single object distance. It does not account for the possibility that different parts of the object could be at different object distances. The approach can also be energy inefficient since the out of focus color channels are not used.
Thus, there is a need for electro-optic imaging systems that can better image color objects while addressing some or all of the drawbacks of the current approaches.